What is the secret to getting the proper forearm rotation in the back swing and downswing? What makes it automatic in the swing and where does this rotation take place?
Part of the problem is the question itself.
1. The 'secret' is there is no secret, at least not one that will work for everyone.
2. Proper for who? A fader, a drawer, underplane, overplane etc....
3. Last but not least you must realize that (although the forearm rotation does help in closing the face) you can rotate your forearm without turning your wrist with it, to a varying degree based on personal flexibility.
Remember Brian's signature...."All you have is your hand path, the force along that path, and torque about that path"... The golf swing is a tango of these three forces interacting throughout the swing. The forearm 'secret' you want is merely a variable that will change from person to person based on many many factors.